This invention relates to phosphor screens for color cathode ray tube (CRT's), and more particularly relates to a method for increasing the adherence of such screens to the face panels of the tubes, and to tubes incorporating such screens.
In producing color CRT's for color television and allied display applications, it is customary to form the phosphor screen photolithographically by forming three interlaced patterns of phosphor elements, one for each of the primary colors red, blue and green. This is accomplished by successively exposing and developing three photoresist layers, each containing a different color phosphor, using a single photomask and a single light source. For each exposure, the light source is moved, resulting in three different beam landing areas for each aperture of the photomask. See, for example, U.S. Pat. Nos. 3,140,176; 3,146,368 and 4,070,596.
In forming such phosphor screens, it is known that too little light during exposure results in incomplete polymerization of the photoresist in the phosphor layer, and consequent poor adhesion of the phosphor elements to the face plate of the tube. As beam landing areas become smaller to accommodate the finer pitch screens of present interest for high resolution displays such as computer displays and high definition television displays, it becomes increasingly difficult to produce screens having adequate adhesion. Increasing the intensity of the light source to improve adhesion often results in the unintentional enlargement of the phosphor elements due to spontaneous polymerization beyond the exposed areas.
In U.S. Pat. No. 3,953,621, adhesion of color CRT phosphor screens is improved by increasing the exposure dosage without increasing the intensity of light from the source. This is accomplished by providing a mirror to reflect light transmitted through the phosphor-photoresist layer and the face panel back onto the layer. However, the reflected light tends to scatter beyond the beam landing areas from which it emerged, resulting in relatively little additional exposure in these areas, as well as the undesired exposure of adjacent areas, causing a condition known as "poor wash".
Poor wash occurs because the adjacent areas become insolubilized by the unintentional exposure, and thus cannot be removed by development. The residual phosphor contaminates these areas and consequently leads to degradation of color purity of the resultant display.
In addition, the light which is reflected back to the desired beam landing areas can be of reduced intensity due to losses such as reflection at the panel surfaces and absorption by the panel itself. The absorption loss can be especially significant, since the transmission of the panel is often intentionally reduced (e.g., from 85% to 52% or even 31%) in order to increase display contrast.